This was the first year for this project, which is using a combination of methods to analyze the ion transport properties of lysosomal membranes. Lysosomes are intracellular organelles that serve in most cells as digestive organelles although in some tissues they are used for other functions. Disorders of lysosome function lead to a variety of diseases including neurological dysfunction (lysosomal storage diseases) and osteopetrosis (overcalcification of bone). Lysosomes utilize an ATP-driven proton pump to maintain an acidic luminal pH and facilitate their digestive function. Such a pump can only be effective if accompanied by additional ion transport to dissipate the transmembrane voltage built up by the ATPase. Genetic evidence suggests that this additional ion pathway a ClC-type anion transporter, but functional experiments have not yet demonstrated such a pathway. We seek to identify the ion transport pathways in lysosomes, to characterize their properties, and to identify the responsible proteins. Our initial efforts have demonstrated the presence of a pH-dependent anion transport activity in lysosomal membranes, consistent with the presence of a functional ClC protein. In the upcoming year we will further characterize the functional properties of this transport and attempt to prove that it is mediated by a ClC transporter.